p8 & 9

• Quantity


  Anything that can be given a numerical value
• Magnitude

  Size of a quantity. E.g. a distance of 5 metres has a higher magnitude than 2 metres
• Scalar

  Describes quantities that only have a magnitude (size). E.g. speed (how fast something is moving)
• Vector

  Describes quantities that have a magnitude AND a specific direction. E.g. velocity (speed in a particular direction)
• Force

  A vector quantity. Forces are pushes or pulls that act on an object. Forces have size and direction. Forces are the result of objects interacting with each other
• Contact forces

  For these forces to act, the interacting objects have to be physically touching
• Non-contact forces

  For these forces to act, the interacting objects don't have to be touching (they are physically separate)
• Resultant force

  The single overall force acting on an object. It has the same effect as all the forces acting on the object all together. The resultant force is the vital thing in working out how an object will move
• If there is a resultant force, the object's speed will change; or the shape of the object will change; or the direction of the object will change. If the resultant force is nothing (the forces cancel out), the object will keep doing what it was doing – either not moving at all, or moving along at a steady speed
• Contact forces

  • friction, air resistance, tension, the normal contact force
• Non-contact forces


  • gravitational force, electrostatic force and magnetic force
• Since forces are a vector quantity, it is useful to show their magnitude (size) and direction using an arrow. The arrow points in the direction that the force acts, and its length shows the magnitude
• Scalar quantities


  • Distance
  • Speed
  • mass
  • Temperature
  • Pressure
  • Volume
• Vector quantities


  • Displacement
  • Velocity
  • Acceleration
  • Force
  • Weight
  • Momentum
  • Work
• In real life, there are usually a few forces acting on any particular object. All the forces can be shown with vectors (arrows). When we take all the forces into account, we can draw just one vector arrow to show a single force, which has the same effect on the object as all the other forces acting at once
• If the forces acting on an object are equal in magnitude and opposite in direction, then the resultant force ends up being ZERO. You can say the forces are balanced. A resultant force of zero means that an object's movement will not change
• Free body diagrams

  Used to describe situations where several forces act on an object. Vector diagrams are used to resolve (break down) a single force into two forces acting at right angles to each other

  
• Parallelogram of force

  1. Work out the scale

  2. Copy out one force to scale so it follows on the other force at the same angle as the original

  3. Repeat for the other force. The forces should meet and create a parallelogram

  4. Draw the resultant force between this point and the point the objects act from

  5. Measure the size of this arrow

  6. Use the scale to determine the magnitude of the force

  
• Work done

  The measure of how much energy is transferred when a force makes an object move. Doing work always involves the transfer of energy
• Joule

  The unit joule (J) is how the amount of energy transferred by doing work is measured. 1 joule = 1 newton metre
• Distance

  How far an object moves. It does not include direction, so distance is a scalar quantity
• Displacement

  The distance an object moves from where it started. This is measured in metres. It is a vector quantity, because it includes the direction an object moved
• Friction

  A contact force that results when two objects move past each other. They have to be touching
• If work has been done, energy has been transferred. Work is always done as a result of a force acting on an object. The amount of work can be calculated using W= Fs
• Displacement is different to distance because it involves the direction that an object has moved. The displacement is always measured in a straight line from start to end of a journey, missing out any wiggles along the way
• 1 joule = 1 newton metre. This means that 1 J is the amount of work done when a force of 1 N causes an object to move 1 m
• When objects move, they are almost always moving against a frictional force – so the friction arrow is opposite to the direction of motion. Doing work against friction causes an energy transfer to heat (thermal) energy. This raises the temperature of the object and the surroundings
• Newton's First Law

  Vehicles moving at a constant speed have a driving (push) force exactly equal to the resistive forces (like friction); Velocity (speed and direction) will only change if there is a resultant force acting (so the resultant force is NOT zero). If an object changed direction, it must have been because of a resultant force
• Newton's Second Law

  An object will only change in velocity (accelerate) if there is a resultant force acting on it. The amount of acceleration depends on the resultant force and the mass of the object
• Inertial mass

  Measures how difficult it is to change the velocity of an object. It is defined as the ratio of force over acceleration
• It requires more force to slow down (change the velocity) a lorry compared to a bike. It also requires more force to make a lorry accelerate compared to a car
• Stationary

  Not moving. The velocity is 0
• Newton's First Law

  The law says that if the resultant force on an object is zero: Stationary objects stay stationary, Moving objects keep moving at the same velocity (same speed and direction)
• Inertia

  The tendency of objects to stay at the same speed or stay stationary
• Newton's Second Law

  Objects accelerate if there is a resultant force acting on them. The amount of acceleration is proportional to the magnitude of the resultant force and inversely proportional to the mass of the object
• Proportional

  Just like in maths: if the magnitude of one quantity increases because another quantity increases, they are proportional. The symbol is ∝
• Inversely proportional

  The opposite of proportional: if one quantity decreases because another one increases, they are inversely proportional
• Newton's Third Law

  This law says that when objects interact, the forces they cause to act on each other are equal and opposite
• This law explains why pushing down with your legs makes you jump up (the ground pushes back with the same size force as your push). It also explains why rockets can fly through space: the gases pushing out the back cause the rocket to move forward
• Lever

  A simple system used to transmit a force. Levers can be distance multipliers or force multipliers